Steve Lillebuen

A massive electrical explosion that rocked the town of Morwell was triggered by critical failures in an SP AusNet power line and in the company's back-up safety systems designed to protect the community, a report has found.

More than 80,000 people lost power in April when live transmission lines came crashing down near the town in Victoria's east, sparking several grass fires.

Victoria's energy safety watchdog, in a preliminary report to be released on Friday, traced the cause of the explosion to a short-circuit on a 66,000-volt transmission line.

The fault occured about a kilometre from the Morwell Terminal Station, triggering a major power surge that should have been stopped by SP AusNet's two safety systems.

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''When the two protection systems failed to isolate the power line, it overheated and failed in a number of locations,'' the report said.

''It subsequently fell into another 66kV line and a 22kV line outside the terminal station, and onto infrastructure within the terminal station.''

The resulting explosions in and around the substation lasted more than a minute and electrified several fences, leading to fears of a major fire had it been a hot summer day.

Energy Safe Victoria director Paul Fearon said the combination of a short-circuit and two protection systems failing was rare and dangerous.

''There's clear public safety issues with these events, not to mention the indirect effect of an outage of this size,'' he said.

The watchdog is still probing the maintenance records of SP AusNet, as well as the power company's operations. Investigators have so far attributed the failure of the first back-up system to an incorrect setting and the second failure to a faulty part.

A cause for the initial short-circuit is still being investigated, with engineers so far unable to rule out a faulty part or some other cause, such as an animal or fallen trees.

The preliminary report also ruled out any link between the explosion and the nearby Hazelwood coalmine fire, which had rained ash on power lines, insulators and conductors for weeks.

Firefighters had reported seeing the ash building up, but SP AusNet insisted it regularly cleaned the insulators.

The report found the insulators on the short-circuited power line were intact and clean, eliminating any chance that the ash was linked.